Industrial methods for producing hydrogen fluoride (HF) generally use a reaction for generating hydrogen fluoride (HF) from fluorite (CaF2) and sulfuric acid (H2SO4) (see, for example, Patent Literatures 1 to 3). Among such methods for producing hydrogen fluoride, there is known a type of method using a jacketed preliminary reactor and an externally heated rotary kiln in combination to conduct two-stage reaction steps. In this type of the producing method, it is known that three reactions occur through the respective reaction steps in the preliminary reactor and the rotary kiln (see, for example, Patent Literatures 4 and 5). Such a conventional method for producing hydrogen fluoride will be descried below by referring to FIG. 1.
Firstly, fluorite (CaF2) and sulfuric acid (H2SO4), which has been mixed with oleum and preheated to 100° C., are separately supplied to a jacketed preliminary reactor 1 (e.g. biaxial kneader) at substantially equal molar amounts, and a solid-liquid mixture of them is kneaded while being heated at about 100° C. Under such a relatively low temperature condition, a reaction expressed by the following formula (1) occurs, dominantly.CaF2+2H2SO4→Ca(HSO4)2+2HF  (1)
A conversion ratio of CaF2 at an outlet port of the preliminary reactor may be 40 to 60%. Hydrogen fluoride (HF) generated by the reaction of the formula (1) is mainly contained in a gas phase, and taken out through an induction pipe 3. A residual reaction mixture in a clayey-to-solid state comprising an intermediate product of Ca(HSO4)2 is transferred to an externally heated rotary kiln 5.
In the rotary kiln 5, the reaction mixture is heated to an increased temperature while it rolls and advances in a direction of a rotation axis. The rotary kiln 5 is heated by flowing hot air at about 500° C. through a jacket. A temperature of the reaction mixture is about 100° C. at an inlet port of the rotary kiln 5 communicating with the preliminary reactor 1, and increased towards an outlet port of the rotary kiln 5 locating at an opposed side thereof, and finally reaches to about 300° C. at the outlet port. Under such a high temperature condition, Ca(HSO4)2 in the reaction mixture degrades by a reaction expressed by the following formula (2). As a result, H2SO4 once spent by the reaction of the formula (1) appears again in the form of a liquid matter, and at the same time gypsum (CaSO4) in a solid form is generated as a by-product.Ca(HSO4)2→CaSO4+H2SO4  (2)
Thus generated H2SO4 by the reaction of the formula (2) reacts with unreacted CaF2 existing in the reaction mixture. Under the high temperature condition as in the rotary kiln 5, not the reaction of the above formula (1), but a reaction expressed by the following formula (3) occurs, dominantly.CaF2+H2SO4→CaSO4+2HF  (3)
Hydrogen fluoride (HF) generated by the reaction of the formula (3) is contained in a gas phase and taken out through the induction pipe 3. A residual reaction mixture mainly contains gypsum (CaSO4) as the by-product, and it is taken out of the outlet port of the rotary kiln 5.
As described in the above, the intended hydrogen fluoride can be obtained by the two-stage reaction steps in the preliminary reactor and the rotary kiln.